This invention relates to an image forming apparatus such as a copying machine capable of changing the surface potential of a photosensitive member which is charged uniformly and stably with the use of a scorotron charger.
Conventionally, in a copying machine or like image forming apparatus, a document image scanned by an optical system is introduced to a photosensitive member, to thereby form an electrostatic latent image on the charged surface of the photosensitive member.
In recent years, there has been available an image forming apparatus having a picture mode for forming an image of a picture document in addition to a character mode for forming an image of a normal black-and-white document containing character information.
When the picture mode is set, the surface potential of the photosensitive member is set low in order to emphasize the intermediate gradation compared to a case where the normal black-and-white document is exposed. Accordingly, the image forming apparatus is required to have such a construction as to make the surface potential of the photosensitive member lower in the picture mode than that of the photosensitive member in the character mode.
An existing scorotron charger for charging the surface of the photosensitive member to attain a fixed surface potential will be described with reference to FIG. 8A. A wire 62 formed of tungsten or the like, when a high voltage is applied thereto from a high voltage supply 60, generates a corona discharge toward a photosensitive member 63 to which an opening of a screening electrode 61 formed of aluminum or the like is facing. A zener diode ZD10 is connected between a grid electrode 64 provided between the wire 62 and the photosensitive member 63 and a ground. The surface potential of the photosensitive member is regulated by the grid electrode 64 whose potential is maintained at a zener voltage, thus controlling the photosensitive member 63 to attain a desired surface potential uniformly and stably.
On the other hand, in another existing scorotron charger capable of causing the photosensitive member 63 to have a plurality of levels of surface potential, zener diodes ZD11, ZD12, and ZD13 are switchably connected between a grid electrode 64 and a ground as shown in FIG. 8B. Accordingly, the potential of the grid electrode 64 can be changed according to the level of the zener voltages by switching the connection, and therefore the photosensitive member 63 is controllably charged uniformly and stably so as to attain the desired level of surface potential.
In this way, the image forming apparatus including the picture mode is required to control the potential of the grid electrode at a multitude of states. Thus, such an image forming apparatus has suffered the disadvantages that a circuit construction for the charger becomes complicated, resulting in an increase in the number of components and in the price of the apparatus itself.